The present invention principally relates to a magnet type, eddy current reduction braking system for assisting frictional brakes in large vehicles.
According to a generator type reduction apparatus (retarder) disclosed in Japanese Patent Application Laid-Open No. 60-255050 Publication, an annular stator body supports a field coil of an induction type generator and is rotated in a direction oppositely to an annular rotor body which supports an armature coil of the induction type generator. The resultant increase in the relative number of revolutions of the annular rotor body with respect to the annular stator body enhances braking performance. Braking energy is recovered as an electric power which is converted into and disposed of as heat in a load resistor or the like. Therefore, braking performance is not reduced by heat generation in the annular rotor body.
However, even if the above-described reversing mechanism is applied to a magnet type eddy current reduction braking system such that a magnet support tube is rotated in a direction reversed to that of a brake drum, cooling performance is not enhanced because the absolute number of brake drum revolutions remain unchanged. Furthermore, since the relative number of revolutions of the brake drum and the magnet support tube increases, braking performance is diminished substantially by increased heat generation in the brake drum.
In conventional magnet type eddy current reduction apparatus, rotational energy of a brake drum is converted into and disposed of as heat produced by eddy currents. However, at present, braking performance achieved is inferior to other reduction systems, particularly fluid type reduction systems.
The braking performance of a magnet type eddy current reduction apparatus is proportional to the magnetic force provided by the magnets, the diamater of the brake drum, and the rotational speed of the brake drum, and is different depending on the material used for the brake drum. However, the diameter of the brake drum is restricted by available space in the vehicle, and the rotational speed of the brake drum is controlled by the rotational speed of a drive shaft to which the brake drum is coupled. Braking performance is enhanced, therefore, only by increasing magnetic strength of the magnets or selection of a different material for the brake drum. However, when braking performance is enhanced, heat generation in the brake drum increases, and is not readily dissipated by cooling fins thereby compromising braking performance improvement. Thus, braking performance cannot be effectively enhanced unless heat transfer out of the brake drum is increased.
The object of the present invention, therefore, is to provide an improved magnet type eddy current reduction apparatus capable of enhanced braking performance.